Remove qualtype qualifier in coroutine error to prevent assert in debug

Summary:
A forward-declared coroutine_traits should trip an error; we need
a complete type.

Unfortunately, in debug mode only, we trip an assert when attempting
to provide the fully qualified type for the error message.
If you try to compile a program with a forward-declared
coroutine_traits in debug mode, clang will crash.

I've included a test for the behavior and removed the q modifier
on the error message. This prevents the crash in debug mode and
does not change the behavior for the error message on a
forward-declaration of a coroutine_traits type.

Test Plan:
I've included a test for the forward-declaration.

Patch by Tanoy Sinha!

Reviewers: modocache, GorNishanov

Reviewed By: modocache

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49099

llvm-svn: 336748